The STEPS Study aims to search for the precursors and causes of problems in child health and well-being by using a multidisciplinary approach. The cohort consists of all mothers (Finnish or Swedish speaking) who had live deliveries in the Hospital District of Southwest Finland from January 2008 to April 2010 and their children (n=9811 mothers, n=9936 children). Of these, 1797 mothers and their 1827 children were recruited to an intensive follow-up group during the first trimester of pregnancy or soon after delivery. Information about the whole study cohort is based on pregnancy follow-up data from maternity clinics, National Longitudinal Census Files and child welfare clinics. Data from multiple sources are used to obtain a picture of the overall well-being of the child and the family. After birth, study visits include several clinical examinations. Collaboration is encouraged, and access to the data will be available when the data set is complete.